Liposomes are unilamellar or multilamellar lipid vesicles which enclose a three-dimensional space. The lipid membranes of liposomes are formed by a bimolecular layer of one or more lipid components having polar heads and non-polar tails. In an aqueous solution, the polar heads of one layer orient outwardly to extend into the aqueous solution and to form a continuous, outer surface. Unilamellar liposomes have one such bimolecular layer, whereas multilamellar vesicles generally have a plurality of substantially concentric bimolecular layers arranged rather like an onion.
Liposomes are well recognized as useful for encapsulating therapeutic agents, such as cytotoxic drugs or other macromolecules capable of modifying cell behavior, and carrying these agents to in vivo sites. For example, U.S. Pat. No. 3,993,754, inventors Rahman et al., issued Nov. 23, 1976, discloses an improved method for chemotherapy of malignant tumors in which an antitumor drug is encapsulated within liposomes and the liposomes are injected into an animal or man. U.S. Pat. No. 4,263,428, inventors Apple, et al., issued Apr. 21, 1981, discloses an antitumor drug which may be more effectively delivered to selective cell sites in a mammalian organism by incorporating the drug within uniformly sized liposomes. Thus, drug administration via liposomes can have reduced toxicity, altered tissue distribution, increased drug effectiveness, and an improved therapeutic index. Liposomes have also been used in vitro as valuable tools to introduce various chemicals, biochemicals, genetic material and the like into viable cells.
However, a deficiency of liposomal drug delivery has been the inability to quantitatively or selectively direct the liposomes' contents to specific sites of action over a therapeutically meaning time frame.
It has been suggested that target, or site, specificity might be conferred on liposomes by their association with specific antibodies or lectins. Methods of associating antibodies with liposomes have been described and may be generally divided into two groups--nonspecific association and covalent attachment.
Non-specific association appears to rely upon the affinity of the Fc portion of the antibody for the hydrophobic region of the lipid bilayer. This has little practical value because the liposomes are rendered more permeable to their encapsulated contents and may themselves be aggregated. Further, it is not believed that this complex would be sufficiently stable in plasma for the considerable periods of time believed necessary in many potential clinical applications.
Considerable effort has ensued in attempts to covalently attach protein to liposomes, with several promising results. For example, Heath et al., have reported efficiently covalently binding liposomes to biologically active proteins by periodate oxidation of glycosphingolipids. Science, Vol. 210, pp. 539-541 (1980). This method of liposome-antibody conjugation has bound up to about 200 .mu.g of protein per .mu.mole of total lipid.